technology, management of DNA typing data, and legal, societal, and ethical issues surrounding DNA typing. The techniques of DNA typing are fruits of the revolution in molecular biology that is yielding an explosion of information about human genetics. The highly personal and sensitive information that can be generated by DNA typing requires strict confidentiality and careful attention to the security of data.

DNA, the active substance of the genes, carries the coded messages of heredity in every living thing: animals, plants, bacteria, and other microorganisms. In humans, the code-carrying DNA occurs in all cells that have a nucleus, including white blood cells, sperm, cells surrounding hair roots, and cells in saliva. These would be the cells of greatest interest in forensic studies.

Human genes are carried in 23 pairs of chromosomes, long threadlike or rodlike structures that are a person's archive of heredity. Those 23 pairs, the total genetic makeup of a person, are referred to as the human "diploid genome." The chemistry of DNA embodies the universal code in which the messages of heredity are transmitted. The genetic code itself is spelled out in strings of nucleotides of four types, commonly represented by the letters A, C, G, and T (standing for the bases adenine, cytosine, guanine, and thymine), which in various combinations of three nucleotides spell out the

FIGURE 1 Diagram of the double-helical structure of DNA in a chromosome. The line shown in the chromosome is expanded to show the DNA structure.